This invention relates to electrical capacitors, particularly for high power applications, that include a plurality of capacitor sections that are individually wound and stacked together in an enclosure with electrical interconnections therebetween and to the terminals of the enclosure.
A capacitor unit for relatively high power applications normally comprises within a single enclosure a plurality of capacitor sections that are mutually interconnected in parallel and/or series combination to achieve the desired capacitance. In conventional commercial practice the capacitor sections are formed individually by rolling on a mandrel sheets of dielectric material and electrode foil material. During the winding process, tabs of conductive material are inserted adjacent the foil electrodes so that merely by pressure contact with the electrodes a conductive interconnection is made between sections. A description of such practice is, for example, contained in copending Application Ser. No. 092,869, filed Nov. 8, 1979 and assigned to the present assignee by Holtzman, now U.S. Pat. No. 4,307,434, issued Dec. 22, 1981, which is herein incorporated by reference. It is apparent that the practice of such technique requires a certain minimum pressure or force to be applied in the wound section to establish reliable electrical contact between the conductive tabs and their adjacent foil electrodes.
It has heretofore been known, see for example Yagatani et al application Ser. No. 724,295, filed Sept. 17, 1976, now abandoned, and Japanese Utility Model Patent Publication No. 8,917/1969, published Apr. 11, 1969 by Iwama et al, that improved electrical stress handling capability can be achieved by the utilization of a rolled edge foil as at least one of the capacitor electrodes. Advantageous use of this technique has been achieved, particularly in combination with a second wide and unrolled edge foil. However, certain problems in the practical manufacturing of such capacitors may result because of dimensional variations that occur across the length of the wound sections.
It has been proposed to have electrodes with folds at the edges with the electrode material folded completely over itself so the section thickness remains uniform throughout, for example as disclosed in published Japanese Published patent application 45-6359 of Mar. 30, 1970 by Iwama et al. Such an arrangement introduces some fabricational complexity in handling the extra folded material as well as extra cost for that material. We are here concerned with rolled edge foil electrodes that result in an increased thickness over a short distance at the electrode edges, such as about 0.375 in. at each edge of a section of about 20 in. in width (i.e. from end to end of the wound section), while the intermediate portion of the section has only a single thickness of each of the foil electrodes.
A growing interest in the art has developed to make capacitors with maximum usage of plastic film material such as polypropylene. In some instances such a film is used as a dielectric spacer in combination with one or more sheets of capacitor grade paper. The combination of the film with paper has the desirable aspect of facilitating thorough impregnation of the entire section, including the film, as the paper layer tends to act as a wick carrying fluid to the interior of the section. While effective capacitors are made employing combinations of film and paper, it is increasingly desirable to maximize the quantity of film in relation to paper, and preferably to avoid the use of paper altogether to provide an all film capacitor, in order to achieve a reduction of size and cost in comparison with equivalently rated film-paper units. The use of the rolled edge technique contributes to the achievement of greatest rating from the smallest size sections. Where a capacitor's dielectric spacer comprises a relatively large amount of film in comparison to paper, and particularly one which is of all film, there is a practical drawback on the ability to achieve thorough impregnation with rolled edge electrical foils.
The extra thickness resulting from the folded or rolled foil edge accumulates in the rolled section so there is a substantial and noticeable greater thickness of the section at the rolled edge than at the center of the section. This was found unobjectionable in making capacitors in which the dielectric spacer was a composite of a layer of film, a layer of paper, and a layer of film. However, in going to all film capacitors, a wound section formed in the previous manner was found to be subject to a practical difficulty. If the tension on the sheet materials being wound was maintained, in an all film winding, the same as that previously used in a film-paper-film winding it was found that finished units after impregnation when tested, by subjecting them to overvoltages, exhibited some failures. When such test units were opened and the sections examined internally, it was found that the liquid impregnant had not reached all portions of the capacitor sections and that particularly areas of the film layers at or proximate to the rolled edge were dry or were less than thoroughly impregnated, apparently accounting for electrical failure at those portions.
Thorough study of a number of such test units has developed an understanding that the winding tension applied to the sheets of material formed into the winding was such that the pressure or force on the section at the rolled edge was so great that a constriction occurred that prevented impregnating fluid from penetrating and reaching all parts of the film layers. Acting on this understanding, it was experimentally determined that if the tension on the layers being wound was lessened to an extent that the thorough impregnation of the dielectric fluid was insured, there was resulting impairment of the security of the conductive tabs inserted in the sections for interconnections. Consequently, the problem was then identified as one of insuring that the pressure at the rolled edge did not exceed that which would permit thorough impregnation and that the pressure throughout the section was sufficient to maintain reliable contact of the conductive tabs.
In accordance with this invention, the problem is solved by winding a capacitor section in a manner so that the roll is loose enough to provide for thorough liquid impregnation and completing the section in that manner with the conductive tabs in place but without necessarily their firm adherence within the section. Then as the sections are stacked closely together within an enclosure, or can, to form a finished unit there are placed one or more pieces of sheet material between adjacent sections dimensioned to fill space and apply additional pressure intermediate the portions of the sections at which the rolled edges occur. This compacts the interior section portions in which a substantial portion of the conductive tabs are disposed. The stacked capacitor sections in the finished unit thus have pressure on them that at the electrode edge portions is sufficiently light so that insulating fluid substantially completely impregnates the dielectric layers throughout the sections including the portions in the vicinity of the rolled edge and pressure intermediate the electrode edge portions is sufficiently great so the conductive tabs within the sections are in reliable pressure contact with their adjacent foil electrodes. The increase in pressure in the intermediate portions of the sections is not so great as to interfere with impregnation of those portions.
The capacitor so formed has been found to be successful in retaining the advantage of the rolled edge as far as the electrical stress capability of the unit is concerned because the rolled edge can now be used with assurance of thorough impregnation, even in all film units, while at the same time insuring secure location of the conductive tabs. Yet this results from a simple to practice technique.
As has been indicated above, the practice of the invention is particularly desirable in capacitors in which the dielectric is of all film material or, in general, of material of low porosity compared to capacitor grade paper.
In the practice of this invention, the intermediate sheet material between the sections preferably extends over between about 80% and 90% of the distance between the rolled edge portions of the sections. The sheet material preferably comprises substantially incompressible material such as pressed hardboard or a firm plastic sheet in a thickness that is between about 5% and 10% of the rolled section thickness.
As will be apparent from the following description, the means employed for selectively compressing the intermediate portions of the sections can take forms other than the sheet material described above.